Just a quick update for another patch 2.1 game bug, courtesy of World of Warcraft's ever-vigilant Public Test Realms (PTR) players. Cthon over at the WoW official forums reports that a bug was found while using the  IsUsableSpell ("Power Word: Shield") command:

IsUsableSpell("Power Word: Shield") now only returns "true" if you have a target. It did not do this before 2.1. Other spells that require a target still return "true" if you have them, whether or not anything is targeted.